The present invention relates to multilayer reflective films, particularly to reducing stress in substrates containing multilayer films, and more particularly to depositing a buffer-layer between the substrate and the multilayer films for adjusting the stress or producing near-zero not stress, and hence little or no deformation of the substrate.
Multilayer structures composed of alternating thin layers of materials, such as molybdenum (Mo) and silicon (Si), with vastly different optical properties have proven effective as high-reflectance, near-normal incidence coatings for various applications. The Mo/Si system which has been shown to give a high reflectance (.about.60%) for certain wavelengths (11-14 nm), is of particular technological importance due to its application to high-resolution, multiple-reflection imaging systems now being developed for projection lithography.
Extreme-ultraviolet (EUV) lithography systems require several precisely figured, low roughness optics coated with reflective multilayers. To obtain sufficient throughput and image quality, these multilayer coatings must simultaneously have high reflectance (R.gtoreq.65%) and low residual stress (.sigma..ltoreq.100 MPa).
There is a strong commercial driving force for increased miniaturization in electronic devices and, hence, an EUV lithography tool has significant commercial potential. The performance of an EUV lithography tool is key to its implementation over other competing technologies, and high film stresses and low EUV reflectances degrade the performance of an EUV lithography tool.
Over the past decade or so, numerous publications describe the dependency of EUV reflectance of Mo/Si multilayer mirrors or optics on their fabrication parameters. However, the number of publications addressing the problem of stress in Mo/Si multilayers designed for high reflectance in the EUV wavelength is relatively small. The later publications describe how the stress of Mo/Si multilayer coatings can be reduced to low levels (&lt;100 MPa) by: 1) post-deposition annealing (see Kola et al, Appl. Phys. Lett. 60,3120 (1992) and Kassner et al, J. Mat. Sci. 31, 2291 (1996); 2) variation of the Mo to Si ratio (see Nguyen et al, in Physics of X-Ray Multilayer Structures, Optical Society of America, Washington, DC., 1994, Vol. 6, P. 103; Windt et al, J. Appl. Phys. 78,2433 (1995); and Tinone et al, J. Electron Specrosc. Relat. Phenom, 80,461 (1996); and 3) adjustment of the sputter deposition process such as base pressure or target power (see Windt et al and Tinone et al above). Non-thermal or a thermal processes such as 1) and 2) above may be particularly valuable since it is not currently known if a several hundred degree Celcius annealing process is compatible with the EUV optics/substrates, holder assembly, etc. However, none of these prior efforts involve the fabrication of a high near-normal incidence reflectance (.gtoreq.65%) Mo/Si multilayer coating with a low stress (&lt;100 MPa). No EUV reflection data was shown, except in the above-cited reference Kola et al, where reflectances around 58% were measured, which is significantly lower than 65%, which is necessary for use in EUV lithography, and in Windt et al which showed a reflectance data for a stress reduction from -440 MPa to -280 MPa with a reflectance of 62%.
At this time there has been little work on Mo/Be multilayers (see Skulina et al, Appl. Optics, 34,3727, 1995 and Stearns et al, in Mater. Res. Soc. Symp. Proc. Vol. 382, p. 329, (MRS, 1995), and the stress problem was neither identified nor addressed. High EUV reflectance Mo/Be films have a net tensile stress, and post-deposition annealing processes are not effective in reducing the stress in these films.
An optic (substrate) will deform when a stressed multilayer film is deposited upon it. A viable EUV lithography process will rely on Mo/Si or Mo/Be multilayer films to efficiently reflect light in the 11-14 nm region. Mo/Si and Mo/Be films with high reflectances (&gt;60%) have large film stresses (&gt;400 and &gt;330 MPa respectively), which will deform the optic and potentially degrade the performance of an EUV lithography tool. Thus, there is a need for reducing the stress in the multilayer films without adversely effecting the reflectance of these films. Reflectance is important since the throughput of an EUV lithography system is expected to scale as reflectance.
The present invention provides a non-thermal or athermal approach to producing multilayer reflective films or coatings with high reflectance (&gt;60%) and low stress (&lt;100 MPa), which are particularly applicable for use in an EUV lithography system, for example. The present invention utilizes a buffer-layer between the multilayer film and a substrate.
Buffer layers have been used to tailor the lattice mismatch between layers in epitaxial semiconductor systems, which affect the stress/strain of the overlying thin film; and particular buffer layers have been selected in part because of suitable thermal expansion coefficient value, which can reduce the stress in certain (non periodic) layered film structures that are deposited at elevated temperatures for microelectronic applications. U.S. Pat. Nos. 4,935,385; No. 5,010,375; No. 5,021,360/ No. 5,128,749; No. 5,393,993; No. 5,457,067; and No. 5,523,587 exemplify the prior utilization of buffer layers for epitaxial and/or thermal expansion properties. There has been no prior effort to reduce stress by use of a buffer layer having a sign opposite that of the layers deposited thereon to cancel out the stress.
The present invention involves the use of a buffer-layer between the substrate and the multilayer film, where the buffer-layer is smooth (&lt;0.3 nm rms roughness) and has a stress of sufficient magnitude and opposite in sign to cancel out deformation due to the stress in the multilayer, so as to result in a tunable, adjustable, or near-zero net film stress, and hence result in little or no deformation of the optic or substrate. For example, the magnitude of stress in a multilayer film of +350 MPa is negated by a buffer-layer film having a stress of about -350 MPa, producing a near-zero stress. The invention provides an a thermal or non-thermal method for reducing the stress without a large degradation in reflectance.